[unity] Fixed json vs binary file detection.

spine-unity/Assets/Spine/Runtime/spine-unity/Asset Types/SkeletonDataCompatibility.cs

@@ -173,11 +173,16 @@ namespace Spine.Unity {
 			int i = 0;
 			if (content.Length >= 3 && content[0] == 0xEF && content[1] == 0xBB && content[2] == 0xBF) // skip potential BOM
 				i = 3;
+			bool openingBraceFound = false;
 			for (; i < numCharsToCheck; ++i) {
 				char c = (char)content[i];
 				if (char.IsWhiteSpace(c))
 					continue;
-				return c == '{';
+				if (!openingBraceFound) {
+					if (c == '{') openingBraceFound = true;
+					else return false;
+				} else
+					return c == '"';
 			}
 			return true;
 		}